/*
LeetCode Problem No.021:    https://leetcode.com/problems/merge-two-sorted-lists/
Author:                     zhangyixing1007
Idea:                       recursion
Time:                       0 ms, beat 100%
Space:                      39.4 MB, beat 22.11%
*/

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
 
class Solution {
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        if(l1==null) return l2;
        if(l2==null) return l1;
        if(l1.val<l2.val) {
            l1.next=mergeTwoLists(l1.next,l2); return l1;
        } else {
            l2.next=mergeTwoLists(l1,l2.next); return l2;
        } 
    }
}
